Chromate treatment or chromating of metal surface for corrosion prevention is well known. Chromate treatment was traditionally carried out with an aqueous system, that is, an aqueous solution of chromic acid. Recently, however, non-aqueous chromate treatment, in which a halocarbon solvent is used as the medium, has been prevailing, since it does not produce harmful aqueous waste and thus no large scale waste soution treatment equiment is required. Typical techniques are disclosed in U.S. Pat. No. 3,285,788 (Du Pont), Japanese Laid-Open Patent Publication No. 56-62970 (Tokuyama Soda), Japanese Patent Application No. 59-153028 (Nippon Dacro Shamrock K.K.), etc.
The non-aqueous chromate treatment solution comprises a non-combustible halocarbon (a hydrocarbon some or all of the hydrogen atoms of which are replaced with halogen atoms) containing an alcohol as a solubilizer, in which anhydrous chromic acid is dissolved. The solution may contain a stabilizer for the purpose of decomposition prevention and a reaction promotor if desired.
The non-aqueous chromate treatment has conventionally been carried out by contacting a cleansed metal surface with a chromating solution by dipping, spraying, etc., and immediately drying the metal surface. Usually the treating solution is kept at its boiling temperature, the metallic material which has been heated to that temperature by contact with the treating solution such as by dipping is immediately taken out of the solvent vapor layer and is dried spontaneously or forcedly. In this, there occurs a disadvantage that chromic acid is taken up unevenly on the metal surface, and spots and speckles are formed on the surface. If the surface is washed with a solvent or solvent vapor after contact with the treating solution, the chromic acid which has been taken up on the surface is lost and only a small amount of chromic acid remains. In order to retain a significant amount of chromic acid even if the treated surface is washed, an extremely long time is required for the reaction. It was practically impossible to effect washing with a solvent in the prior art and it was very difficult to control the amount of chromic acid taken up. Therefore, washing is not carried out in the conventional non-aqueous process.
In the course of our study to solve the above-mentioned problems in the non-aqueous chromate treatment, we have found that the chromic acid taken up on the metal surface is not lost and only the chromic acid portions taken up unevenly and excessively are removed and a good finish is attained by allowing the metallic material which has been contacted with the treating solution to stand for a short time in an atmosphere which substantially does not contain vapor of any ingredient of the treating solution before washing. Further we have found that the amount of chromic acid to be taken up can be controlled by regulating the condition under which the treated metallic material is allowed to stay in said atmosphere, and thereby attained this invention.